DESCRIPTION: The objective of this study is to test the hypothesis that bradykinesia in PD can be characterized a disorder of scaling, where the term scaling is taken to apply to movement velocity, movement amplitude and movement force. This hypothesis is made anatomically specific, as follows. It is argued that: 1) in normal subjects movement scalings chiefly controlled by the basal ganglia; 2) in pre-pallidotomy patients scaling control will be cortical; and 3) in post-pallidotomy PD patients, the cerebellum and thalamus will assume dominance. As these hypotheses are anatomically well specified and involve structures at a large remove from one another, it is proposed that PET functional brain imaging be used to test them. This is done by means of a battery of 18 limb-movement conditions in which the demands for velocity, amplitude and force are systematically varied. The same battery of movement tasks (simplified to ten conditions in PD patients, to accommodate physical fatigue) is applied to normal volunteers, pre-pallidotomy PD patients on and off medications, and post-pallidotomy PD patients on and off medications.